Repeatability and Agreement of Central Vault for Implantable Collamer Lens Obtained by the Tomey OA-2000 Biometer and Spectralis OCT

Objective To assess repeatability and agreement of central vault for implantable collamer lens (ICL) measured by the Tomey OA-2000 biometry and Spectralis optical coherence tomography (OCT). Methods In this prospective study, the central vault was measured by the Tomey OA-2000 biometer and Spectralis OCT in 84 eyes (43 patients) after ICL implantation at six month follow-up. Three consecutive scans were obtained by one experienced technician using Tomey OA-2000 and the Spectralis OCT in the same day. The coefficient of variation (CoV), intraclass correlation coefficient (ICC), within-subject standard deviation (Sw), and 2.77 Sw were calculated to assess the repeatability and reproducibility. The paired t-test and Bland–Altman plots were used to analyze the differences and agreements of central vault measured by two devices. Results Repeatability of the central vault measured by Tomey OA-2000 biometer and Spectralis OCT showed that the CoV was 2.71% and 1.66%, respectively. The ICC for both devices was 0.996 and 0.999, respectively. The paired t-test showed that central vault measured by Tomey OA-2000 biometer was −7.25 ± 23.57 microns lower than that measured by Spectralis OCT (P = 0.006). The mean difference between measurements for Tomey OA-2000 and ASM-OCT with 95% limits of agreement (LoAs) was −38.94 to 53.44 μm. Conclusion Both Tomey OA-2000 biometer and Spectralis OCT displayed good repeatability for the measurement of central vault of ICL. Good reliability and agreement were observed between Tomey OA-2000 biometer and Spectralis OCT. Both instruments are useful but not replaced each other for central vault measurements.


Introduction
Myopia is one of the most common eye diseases in the world [1].Tere are more than 2 billion myopic individuals, which include 277 million high myopia patients [2].Implantable collamer lens (ICL) implantation could correct a wider range of myopia and became an important surgical option for myopia and especially for high myopia patients [3].
Many studies reported that ICL implantation was a safe, efective, and even reversible surgical approach for myopia correction.Appropriate ICL central vault is considered to be highly related to the success and safety of ICL implantation.
Generally, a desirable central vault from the central posterior surface of the ICL to the anterior surface of the crystalline lens is about 250-750 μm for V4 model [4].Extra high vault is a potential risk for angle closure and high intraocular pressure, whereas extra low vault could be the reason of the anterior subcapsular cataract (ASC) or cataract.However, in terms of V4c ICL with a central hole, Gonzalez-Lopez et al. found a good long-term tolerance of the crystalline lens and protection against anterior subcapsular cataract to low vaulting compared with V4 ICL without a central hole [5].
Te precise measurement of central vault is highly emphasized.Several anterior segment imaging instruments have been developed to objectively visualize and evaluate central vault.Optical coherence tomography (OCT), Pentacam analysis system, and ultrasound biomicroscopy (UBM) are now widely used [6][7][8].
Tomey OA-2000 biometer is a device based on optical low coherence refectometry (OLCR) which is now commercially available for ocular biometry.Te anterior segment parameters could be automatically measured without the need of realignment [9,10].Several studies have showed excellent repeatability and reproducibility of Tomey OA-2000 for anterior segment ocular parameter measurements [9][10][11].However, Tomey OA-2000 has not been applied for central vault measurement although it is theoretically possible.Spectralis OCT, combined with the anterior segment module (ASM) added on lens and accompanied by a software package build in it, is also ofers high resolution cornea, sclera and anterior chamber angle images [12].However, to the best of our knowledge, it has not been tried for central vault measurement yet.Te purpose of this study was to evaluate agreement and repeatability of central vault obtained by the Tomey OA-2000 biometer and ASM-OCT.

Subjects and Methods
All subjects enrolled in this study underwent V4c (EVO Visian ICL) model implantation for correction of moderate to high myopia with or without astigmatism between September 2019 and March 2022 in the department of ophthalmology of the Tird Hospital of Hebei Medical University.Te point to emphasize is 8 patients younger than 21 years old were involved in this study.All these 8 patients signed the extra content for ICL implanting beyond indications, whom thoroughly understand the complication and accepted this procedure of own volition.ICL implantation operations were performed by the same experienced surgeon (Z.P.Y) according to the regular procedures [8].Te implantable collamer lens power calculations were completed by STAAR Surgical Company using a modifed formula.Te sizes of lens were selected depending on the corneal horizontal white-towhite value and anterior chamber depth measured by Pentacam analysis system (Oculus, Wetzlar, Germany).Te inclusion criteria were patients with central vault between 100 μm and 1000 μm (measured by ASM-OCT) and no surgery-related complication after ICL surgery.All patients were well evaluated during routine follow-up examination.Tis study followed the Declaration of Helsinki and was approved by Ethical Committee Review Board of the Tird Hospital of Hebei Medical University.All subjects provided written informed consent when the purpose of the study was explained to them in detail.
Sample-size calculation was performed a priori with the F-test ANOVA repeated measures method by Gpower 3.1 software.Te parameter settings are as follows: Number of groups = 2, Number of measurements = 3, Corr among rep measures = 0.5, and Efect size f = 0.255.Using a 2-sided level of signifcance α = 0.05 and Power of 1-β = 0.8, the samplesize calculation indicated that a minimum of 84 samples would be required.

Central Vault Measurement by Tomey OA-2000 and ASM-OCT.
A single experienced operator performed 3 consecutive scans at 6 months after ICL implantation with both Tomey OA-2000 biometer (Tomey, Nagoya, Japan) and ASM-OCT (Heidelberg Engineering, Heidelberg, Germany), respectively.To control the bias between 3 consecutive vault measurements, all subjects accept training before examination so as to be well cooperated with good fxation ability when measurement.All measurements were operated in the morning and under the same dim light condition to minimize variations in the results.Patients sat in front of the devices, put their chin on the chinrest, focused on the target accordingly, and opened their eyes wide after blinking before each scan.Te whole procedure was completed within 15 minutes, and only qualifed measurements were adopted.In terms of Tomey OA-2000 biometry, a central averaged depth profle (A-scan) is presented in Figure 1(a).Te averaged A-scan shows light intensity refected from diferent structures of the eye.Te frst two peaks arise from the anterior and posterior surfaces of the cornea (line a and line b).Next two peaks arise from anterior and posterior surface of ICL (line c and line d) and last two peaks arise fromanterior capsule and posterior surface of crystalline lens (line e and line f ) (as shown in Figure 1(a)).In detail, line a, line d, and line f were automatically displayed at the anterior surface of cornea, the posterior surface of ICL, and the posterior surface of crystalline lens, respectively.Line e was manually made at the anterior surface of crystalline lens.Ten the central vault was defned as the distance between line a and line e minus the distance between line a and line d.It is necessary to emphasize that line e should be set up at the correct position.Te precision of the caliper tool of the Tomey OA-2000 is 10 microns.For ASM-OCT, the vault was defned as the central distance from the back surface of the ICL to the anterior surface of the lens capsule (as shown in Figure 1(b)).Te details of measurements by ASM-OCT were similar to those of anterior segment OCTreported from other previous studies [11,13].

Statistical Analysis.
All data were analyzed using SPSS software for Windows version 25 (IBM corporation, USA) and MedCalc statistical software (version 20.027, MedCalc Software Inc, Belgium).Te sample size was analyzed and decided by Gpower 3.1 software.Te distribution of all the datasets was analyzed for normality using Kolmogorov-Smirnov tests (P > 0.05).To determine the repeatability for two instruments, intraclass correlation coefcient (ICC) with absolute agreement, within-subject standard deviation (Sw), and within-subject coefcient of variation (CoV) was calculated for 3 consecutive measurements.Te test-retest repeatability was defned as 2.77Sw, which indicated the interval within which 95% of the diferences between measurements are expected to lie.Te CoV was calculated as the ratio of the Sw to the overall mean.A smaller CoV means that the repeatability was higher.Te ICC (ranging from 0 to 1) assesses the consistency for datasets of repeated measurements.Te closer the ICC is to 1, the better the measurement consistency is, and a value more than 0.9 indicates 2 Journal of Ophthalmology acceptable clinical reliability.To compare central vault obtained by 2 devices, a paired t-test was applied to identify pairs that had signifcant diferences.Bland-Altman plots were constructed to assess the repeatability of measurements with every device and agreement of measurements between 2 devices.Te 95% limits of agreement (LoAs) were defned as ±1.96 standard deviation.A narrower 95% LoA indicated better agreement between measurements.All tests were two tailed analyzed, with P values less than 0.05 considered statistically signifcant.values were greater than 0.05 (0.649 and 0.998) which indicated that there was no signifcant diference in the repeated measures using the same instrument.Tis result showed high repeatability of vault values measured by Tomey OA-2000 or ASM-OCT.

Reliability and Agreement between Tomey OA-2000
Biometer and ASM-OCT.Te average of 3 measurements of vault by the Tomey OA-2000 was used to compare with that measured by ASM-OCT.Te paired t-test showed that the vault value measured by Tomey OA-2000 was signifcantly higher than that measured by ASM-OCT (P � 0.006) (Table 3).Repeatability of three central vault measurements for both Tomey OA-2000 and ASM-OCT was displayed using Bland-Altman plots (Figure 2).Agreement of central vault measurements between two methods was illustrated using Bland-Altman plots (Figure 3).Te mean diference between measurements of Tomey OA-2000 and ASM-OCT with 95% limits of agreement (LoAs) was −38.94 μm to 53.44 μm.Tis result indicates that repeatability and agreements of the central vault measured by two devices were relatively good.

Discussion
One of the important parameters to be assessed after ICL implantation was the central vault [14].Tere are several devices that have been used to measure central vault after ICL implantation, such as anterior segment optical coherence tomography (AS-OCT), Pentacam, and ultrasound biomicroscopy (UBM) [15].All these devices have been proved for well precision (repeatability and agreement) in many previous studies [7,8].4

Journal of Ophthalmology
Te Tomey OA-2000 biometer, which based on the principles of SS-OCT (Swept-Source Optical Coherence Tomography) and Placido disk topography, was designed for measuring axial length and anterior segment parameters including central corneal thickness (CCT), ACD, and lens thickness [16].Although Tomey OA-2000 biometer could not calculated central vault values automatically like as other anterior segment parameters, A-scan profle images achieved by Tomey OA-2000 clearly displayed six peaks arised from anterior and posterior surface of the cornea, ICL and the crystalline lens.As a result, it could be used for central vault masurement.Te central vault between peak line d (the posterior surface of ICL) and peak line e (anterior capsule of crystalline lens) was measured manually by one experienced technician.Tat the ICC value(>0.98)for central vault measured by Tomey OA-2000 displayed highly repeatability, which was comparable with other ocular parameters measured by this instument (ICC = 0.91-1.00)[8].As a result, it should be one choice for central vault evaluation in clinics.Subsequently, we choose ASM-OCT for central vault measurement and compared it with Tomey OA-2000 for agreement examination.
Te Spectralis anterior segment module (ASM) is an add-on lens, accompanied by a software package that can be added to the Spectralis SD-OCT (spectral domain OCT) device.ASM-OCT was diferent from anterior segment OCT (AS-OCT) which is being routinely accepted as the method of choice for central vault measurements [17].Te SS-OCT technology with longer wavelength light sources (1300 nm) delivers high-resolution images of the anterior segment along with a large image depth, at a fast acquisition speed, but ASM-OCT is SD-OCT using shorter wavelength light sources (820-880 nm), which is allowed for anterior segment imaging by special module.Although it cannot show entire anterior segment in one image like AS-OCT displayed, the ASM-OCT can ofer similarly high-resolution images of the cornea, sclera, and anterior chamber angle, respectively [18].Intriguingly, to the best of our knowledge, there is no study reported for central vault measurement with ASM-OCT.In fact, it can display the perfect ICL contour in the anterior segment of the eye after implantation.Te current study proved that the pretty high repeatability (ICC more than 0.99) for central vault measured by ASM-OCT, but images capture need longer study curve for technician.Although lack of comparison data between ASM-OCT and AS-OCT, Similar scanning images suggested that ASM-OCT was comparable with the classical AS-OCT in terms of repeatability of central vault measurement [8].In this study, the repeatability values (2.77 Sw) for both devices were 28.016 and 16.775 microns.Because a desirable vault height is between 250 and 750 microns, the diferent values between 2 consecutive vault measurements are clinically meaningful, especially in patients with low vault values.For that matter, both AS-OCT and Tomey OA-2000 may not be fully competent for postoperatively evaluating the safety of myopia patients after ICL implantation, especially for measuring low vault.
To the best of our knowledge, there is no study to investigate the precision of central vault measured by two devices.Tis study frst evaluated the precision of central vault measured by Tomey OA-2000 and ASM-OCT and found excellent repeatability.In terms of LoAs values and P values, two devices displayed poor agreement and could not be used interchangeably for evaluating the central vault after ICL implantation.Central vault values with OA-2000 were signifcantly lower than that with ASM-OCT by a mean of 7.25 microns.In fact, it is not uncommon that there are statistically signifcant for anterior segment parameters measured by diferent devices [7,8].Wan et al. reported that AS-OCT showed higher central vault values than that of Pentacam system and UBM, while Pentacam showed lower measurements than that of UBM [8].Several reasons might explain the diference among two instruments in current study.Firstly, Tomey OA-2000 and ASM-OCT have different resolution and lead to the diferent measured values.Secondly, the diference of scan location may contribute to the disagreement among instruments.Te central vault also cauld be dynamic changed with pupil movement under diferent background illumination.Besides, the diferent refractive indices between ICL material and aqueous humor would have efect on the measurement of all structures lying behind the ICL.In terms of the axial revolution, OA-2000 and Spectralis OCT was 10 microns and 7 microns respectively [13], so a 6-micrometer diference with a LoA of below 50 microns seems a small diference for both devices [19].Tat result demonstrated the vault values measured by two devices are pretty close although they cannot replace each other for central vault measurements.In brief, both ASM-OCT and Tomey OA-2000 could provide objective ways for central vault measurements after ICL implantation, but that could not determine which one was more accurate as lacking of a gold standard measurement so far Tere are some limitations in this study.First, this study only focused on Chinese eyes, sample size was small, and more subjects are needed for further examination.Second, this study only explored Tomey OA-2000 and ASM-OCT for central vault measurements; other popular instruments including AS-OCT and UBM were not involved for comparison.Further research on the consistency of multiple instruments is required.Due to the changes in pupil diameter depending on diferent light conditions could induce the dynamic variations in vault, [19][20][21] the measurement of the vault was not in the same light condition is another limitation of this study.Additionally, a learning curve is necessary becaused central vault is manully measured on Ascan profle by Tomey OA-2000.Tere is also no specifc build-in modulation available for central vault measurement for both devices.

Conclusions
Tis study reported the novel application of Tomey OA-2000 and ASM-OCT for central vault measurement after ICL implantation.Both devices displayed highly repeatable for central vault measurement.However, two devices displayed poor agreement and could not be used interchangeably for evaluating the central vault after ICL implantation.

Figure 1 :
Figure 1: Vault measurements made using the OA-2000 biometer (a) between the anterior surface of the crystalline lens (line d) and the posterior surface of the ICL (line e); line a: anterior surface of the cornea; line b: the posterior surface of the cornea; line c: the anterior surface of the ICL; line d: the posterior surface of the ICL; line e: the anterior surface of the crystalline lens; line f: the posterior surface of the crystalline lens.Central vault measurements made using the ASM-OCT (b).Vault was measured between the anterior surface of the crystalline lens and the posterior surface of the ICL.

Figure 2 :
Figure 2: Bland-Altman plots displayed repeatability for three vault measurements by OA-2000 and ASM-OCT, respectively.Reading 1 � minimum of three measurements of vault; reading 2 � maximum of three measurements of vault.Reading 2 − reading 1 � range.Te vertical axis represents the diference between these measurements and the horizontal axis shows the corresponding mean value.Te 95% LoA is indicated using dashed lines, and the middle bold line represents the mean diference between these measurements.

Figure 3 :
Figure 3: Bland-Altman plots comparing the level of agreement between OA-2000 and ASM-OCT measurements of vault.Te vertical axis represents the diference between these measurements and the horizontal axis shows the corresponding mean value.Te 95% LoA is indicated using dashed lines, and the middle bold line represents the mean diference between these measurements; A � average of three vault measurements.
OA-2000: OA-2000 biometer; ASM-OCT: anterior segment module of Spectralis OCT; ICC: intraclass correlation coefcient; vault: distance between the back surface of ICL and the front surface of the lens.P values less than 0.05 were considered statistically signifcant.

Table 3 :
Comparison of vault measurements using OA-2000 and ASM-OCT (n � 84).* Paired samples test was used to compare measurements.P values less than 0.05 were considered statistically signifcant.Asterisk means the vault value was signifcantly diferent between OA-2000 and ASM-OCT.